1. Field of the Invention
The present invention relates to telephoto optical lens systems and more particularly to an improved focusing mode of operation.
2. Description of the Prior Art
In the general field of photographic lens systems, focusing has been frequently accomplished by the movement of the entire lens element assembly. This is sometimes a relatively easy expedient for focusing in smaller compact lens assemblies of a fixed focal length.
However, in a telephoto lens assembly, its diameter, overall length and weight are by necessity increased over both a standard and wide angle lens systems. Accordingly, any focusing arrangement which requires the movement of the entire telephoto lens system would also require mechanisms that are by necessity relatively complex, strong and durable. Additionally, the moving distance for focusing is also necessarily increased. Needless to say these requirements all contribute to a higher cost for the lens barrel and produce both increased weight and size.
Modern photographic demands have required more compact and lighter telephoto lens systems. As any photographer knows, the relative weight of a telephoto lens system makes this system extremely hard to be held stable and adds human physiological vibrations that are introduced into the image plane of the camera.
Optical designers, when working with a telephoto lens assembly design have attempted to simplify focusing by moving a rear lens component. These rear lens components are generally relatively small in diameter and of light weight, especially compared with the more conventional systems wherein the entire lens assembly are moved as a unit. By utilizing this design approach, a movable rear lens component in a telephoto lens system generally has the advantage of a reduced lens movement, less barrel manufacturing cost and greater ease of operation. This design approach, however, suffers a disadvantage in that a variation in aberrations, due to movement of this rear lens component, are greater than in the conventional system. Particularly, in a relatively high speed telephoto lens system, the variation in aberrations caused by close focusing by means of moving the rear lens component are serious, and the bulkiness of the whole lens system is apt to be increased.
Typical examples of such focusing systems are described in U.S. Pat. No. 4,240,703 and in the following Japanese Patent applications that have been laid open to inspection to the public, Japanese Patent Application Laid Open No. 46038/1979 and Japanese Patent Application Laid Open No. 147606/1980.
Thus, there is still a demand in the prior art to provide an improved mode of focusing in telephoto lens assemblies.